Method for electrolytic water disinfection without cathodic hydrogen evolution

ABSTRACT

The invention relates to a novel device for electrolytic disinfection of drinking water, service water and waste water using anodically generated disinfectants. The cathodic formation of hydrogen is prevented by using gas diffusion electrodes as the cathode. Atmospheric oxygen is reduced to hydroxyl ions and/or hydrogen peroxide at the gas diffusion electrodes. A permanent anode can be positioned between two gas diffusion electrodes. The latter then function alternately as a cathode or a second anode. This polarity change enables the removal of the deposits containing metal ions that form during the cathodic reaction from the gas diffusion electrodes. The substances with the disinfecting effect are produced through the use of different electrode materials for the anode and the diffusion electrode either at both anodes or at the anode and cathode.

CROSS-REFERENCE TO RELATED APLICATIONS

This application is a continuation of, U.S. patent application Ser.No.10/381,756 filed Mar. 26, 2003, which is based from InternationalPatent Application No. PCT/EP01/11287, International Filing Date 27 Sep.2001.

BACKGROUND OF THE INVENTION

The invention relates to a device for electrolytic disinfection ofdrinking water, service water and waste water, using anodicallygenerated disinfectants, wherein the undesired formation of hydrogenduring the partial cathodic reaction is avoided.

Electrolytic water disinfection is an efficient and cost-effectivemethod of disinfecting water. It can be defined roughly as follows,killing of micro-organisms in water to be treated, through the action ofan electric current introduced into the water via electrodes. Thiselectric current can lead both to the anodic generation of disinfectantsubstances from the water itself or from substances dissolved in it, andalso to the direct killing of micro-organisms on contact with theelectrodes and by shifts of the pH value in the vicinity of theelectrodes.

The most important anodic reactions with which oxidising anddisinfectant substances (e.g. hypochlorous acid HClO, peroxodisulfateS₂O₈ ²⁻, hydrogen peroxide H₂O₂, ozone O₃, permanganate MnO₄ ⁻) aregenerated from the water and its natural components, are the following:2 Cl⁻→Cl₂+2 e ⁻  (1 a)Cl₂+H₂O→HClO+HCl   (1 b)2 HSO₄ ⁻→S₂O₈ ²⁻+2 H⁺+2 e ⁻  (2)2 H₂O→H₂O₂+2 H⁺+2 e ⁻  (3)3 H₂O→O₈+6 e ⁻+6 H⁺  (4)Mn²⁺+4 H₂O→MnO₄ ⁻+8 H⁺+5 e ⁻  (5)

Generally, the hypochlorous acid (also described as so-called “freechlorine”) generated from the natural chloride content of the wateraccording to equations 1 a and 1 b is by far the most importantdisinfectant of electrolytic water disinfection.

In addition to the reactions according to equations 1 to 5, oxygenevolution takes place as the anodic reaction, generally in great excess.2 H₂O→O₂+4 H⁺+4 e ⁻  (6)

Besides the designations electrochemical or electrolytic waterdisinfection, other names are also often used, such as the somewhatmisleading designations Anodic Oxidation or Weak-current Electrolysis.

A large number of devices for electrolytic water disinfection are knownaccording to prior art (e.g. the publications DE3430616, EP515628, U.S.Pat. No. 5,807,473, EP711730, DE19534736, WO97/11908, DE19633342), butall of these have the decided disadvantage that during the electrolytictreatment of the water to be disinfected, hydrogen is produced at thecathodes in accordance with2 H₂O+2 e ⁻→H₂+2 OH⁻  (7)

If the cathode reaction runs 100% as hydrogen evolution, roughly 0.4 lhydrogen are produced per Ah. In particular, in cooperation with oxygenproduced during electrolysis at the anode in accordance with equation(6), the hydrogen can lead to the formation of a dangerous explosiveoxyhydrogen gas mixture.

Furthermore, the accumulation of quite large amounts of hydrogen inpipeline systems in which water circulates can lead to disruption ofwater circulation. Hydrogen can penetrate into many metals and lead tospecific forms of corrosion or embrittlement. For these reasons, theundesired formation of hydrogen has been up to the present day the mostimportant cause for the method of electrolytic water disinfection notbeing able to become established on the market to a greater extent.

In DE19631842 is proposed the use of an oxygen-consuming cathode for theelectrolytic treatment of drinking water and service water. At anoxygen-consuming cathode, oxygen can be reduced to hydrogen peroxide andhydroxyl ions according to equation (8), or only to hydroxyl ionsaccording to equation (9).O₂+2 H₂O+2 e ⁻→H₂O₂+2 OH⁻  (8)O₂+2 H₂O+4 e ⁻→4 OH⁻  (9)

The use of such an electrode does indeed lead to the avoidance ofcathodic hydrogen formation, if sufficient oxygen is available to beable to let the reaction of the oxygen reduction run its desired course.Moreover, even during the cathodic reaction, a disinfectant is formed,and this possibly improves the efficiency of the method. However,hydrogen peroxide has a lower oxidation potential by comparison withmost of the substances formed at the anode according to equations 1 to5, and therefore often only leads to unsatisfactory results. Also,hydrogen peroxide is not permitted in every case as a disinfectant.Thus, for example, in Germany it must not be used for disinfectingdrinking water. What is also disadvantageous is that the hydrogenperoxide formed at the oxygen-consuming cathode can react with theanodically formed free chlorine according toH₂O₂+HClO→H₂O+HCl+O₂   (10)and this can lead to the elimination of the most important disinfectantformed anodically, free chlorine. A further disadvantage ofoxygen-consuming cathodes of the type mentioned in DE19631842 is thefact that only the oxygen dissolved in the water can be used for thereactions as per equations 8 and 9. The solubility of oxygen in water isvery low, however, and at atmospheric pressure is typically at the most8 to 10 mg/l. Even if pure oxygen from a gas cylinder or the anodicallyformed oxygen is used to saturate the water with oxygen, only valuesbelow 25 mg/l are obtained. Therefore, the possible reaction speeds andconsequently the applicable current densities are very low, if one doesnot wish to accept the cathodic formation of hydrogen. For this reason,electrolytic devices for water disinfection using oxygen-consumingcathodes according to DE19632842 have also not been able to becomeestablished to a greater extent in practice.

The object underlying the invention, therefore, is to quote anelectrochemical device which is substantially improved by comparisonwith the prior art and with which electrolytic water disinfection usinganodically generated disinfectants and avoiding cathodic hydrogenevolution can be carried out rapidly, reliably and in a cost-effectivemanner.

SUMMARY OF THE INVENTION

According to the invention, the object is accomplished in that as thecathodes are used oxygen-consuming cathodes in the form of gas-diffusionelectrodes, at which oxygen, preferably atmospheric oxygen, is reducedto hydroxyl ions and/or hydrogen peroxide. In these gas-diffusionelectrodes, oxygen diffuses from the ambient air through awater-impermeable but oxygen-permeable membrane into a porous electrodematerial.

In this porous electrode material, which is also penetrated by theelectrolyte, i.e. the water to be disinfected, the oxygen is thenreduced to hydroxyl ions or hydrogen peroxide and hydroxyl ions. In bothcases, hydroxyl ions are produced which leads to an alkaline pH value inthe immediate vicinity of the gas-diffusion electrode connected as thecathode.

In an embodiment of the invention, the gas-diffusion electrode comprisesa water-impermeable but oxygen-permeable Teflon film, a carbon layeracting as the porous electrode and a metal wire netting or expandedmetal which serves for supplying current and for the mechanicalstabilization of the electrode.

According to a further feature of the invention, the metal wire nettingor expanded metal consists of titanium or some other valve metal sincethese have a particularly high electrochemical stability.

In various applications of electrolytic water disinfection, the cathodicformation of hydrogen peroxide can be used very sensibly to supplementthe anodic formation of disinfectants. In applications in which theformation of hydrogen peroxide is not desired, such as in the case ofdisinfecting drinking water for example, the cathodic formation ofhydrogen peroxide at the oxygen-consuming cathode can be avoided,according to an embodiment of the invention, in that the carbon layer iscoated with platinum. In a further embodiment of the invention, insteadof platinum the oxide of an element from the group of platinum metals,preferably iridium oxide or ruthenium oxide can be applied for thispurpose to the carbon layer or/and to the metal wire netting or expandedmetal.

If water at relatively high pressures, such as are usual in the domesticwater supply for example, is to be treated by electrolytic waterdisinfection, the electrodes used must also permanently withstand thesepressures. According to the invention, the gas-diffusion electrode mustbe additionally supported from outside in order also to be able to treatelectrolytically water at quite high pressures.

In an embodiment of the invention, titanium electrodes coated with mixedoxides are used as the anode. Titanium electrodes coated with mixedoxides are particularly suitable when hypochlorite and hypochlorous acidare to be generated as effectively as possible from the natural chloridecontent of the water.

In a further embodiment of the invention, diamond electrodes doped withboron are used as the anode. These boron-doped diamond electrodes areespecially suitable when the natural chloride content of the water to bedisinfected is very low and other disinfectant substances, such asozone, peroxodisulfate and in particular OH radicals, for example, areto be generated as effectively as possible.

In the treatment of water which contains constituents which form poorlysoluble precipitates and deposits, the unavoidable formation of hydroxylions during oxygen reduction leads to deposits on the cathode. Anexample is the deposition of lime on the cathode during theelectrochemical disinfection of water containing hardening constituents.The time is usually removed by a periodic change of polarity. It isknown, however, that the service life of titanium electrodes coated withmixed oxides is severely reduced by a periodic change of polarity.Moreover, a change of polarity when a gas-diffusion electrode and atitanium electrode coated with mixed oxides are being used would againlead to the production of the undesired hydrogen gas occurring when thetitanium electrode coated with mixed oxides is connected as the cathode.In an embodiment of the invention, therefore, a unit of the device forelectrolytic water disinfection avoiding cathodic hydrogen evolutioncomprises an anode which is positioned between two gas-diffusionelectrodes, only one of the gas-diffusion electrodes being connected asthe cathode, but the second being connected as an auxiliary anode.

According to the invention, a periodic change of polarity takes placebetween the two gas-diffusion electrodes in order to dissolve againanodically deposits formed on the cathode. The anode located between thegas-diffusion electrodes operates by contrast as a permanent anode.

In an embodiment of the invention, a modular device which can be adaptedto a particular problem is produced in that a plurality of unitscomprising respectively two gas-diffusion electrodes and an anodelocated between same are connected in parallel or in series behind oneanother.

BRIEF DESCRIPTION OF THE DRAWINGS

In FIGS. 1 and 2 are shown diagrammatically possible embodiments for theconstruction of a device for electrolytic water disinfection usinganodically generated disinfectants without cathodic hydrogen evolution.

DESCRIPTION OF PREFERRED EMBODIMENTS

1. FIG. 1 a is a sectional view of a complete electrolytic cell. Thecell follows the principle of a frame-type pressure cell. Between twopressure plates (4 a and 4 b) are fixed an anode (2) and a gas-diffusionelectrode as the cathode (1). The pressure plate 4 b (FIG. 1 b) is hereperforated over the area of the gas-diffusion electrode. Thegas-diffusion electrode is mechanically stabilised over the entiresurface by a porous support plate (3). A porous filter material (40%porosity) of pure polyethylene was used as the support material.Unrestricted access of air to the gas-diffusion electrode is possible.Water flows through the cell from top to bottom.

a) For the electrolytic generation of free chlorine as a disinfectant, atitanium electrode coated with iridium, produced by the company MetakemGmbH Usingen, is used as the anode. The gas-diffusion electrodecomprises a metal wire netting or expanded metal (e.g. Ni, Fe, Ti) and agraphite layer with catalysts (e.g. Mn, Pt) which prevent the productionof hydrogen peroxide.

Tests were carried out with various gas-diffusion electrodes and varyingthe chloride content of the water and the current density. When thegas-diffusion electrode MOC (with PTFE on silver-plated nickel netting;from the company Gaskatel GmbH Kassel) is used, 33 mg/Ah free chlorinecan be obtained with a chloride content of the water of 60 mg/l withj=18 mA/cm.sup.2 (further values in Tab. 1). Production of free chlorineChloride concentration in Current density Current density the water j =18 mA/cm² j = 38 mA/cm²  60 mg/l  33 mg/Ah  35 mg/Ah 240 mg/l 160 mg/Ah250 mg/AhThe pH value and the electrical conductivity of the treated water remainunaltered.

b) For the anodic generation of hydrogen peroxide as a disinfectant,boron-doped diamond electrodes (Fraunhofer Institut Schicht- andOberflchentechnik Braunschweig) are used as the anode. As thegas-diffusion electrode are used the kinds which have hydrogenperoxide-active types of graphite. Various concentrations of hydrogenperoxide can be obtained via the selection of the types of graphite andthe current density. Description of the types of Hydrogen peroxide Pos.graphite production (mg/Ah) 1 Graphite KS 75 112 2 Graphite MCITB 25 3Graphite PC 006 205

2. FIG. 2 is a sectional view of a cell for electrochemical waterdisinfection and treatment, in which a periodic change of polarity ispossible between the gas-diffusion electrodes. The cell is based on theprinciple of a frame-type pressure cell. Between two gas-diffusionelectrodes (1), which are stabilised over the entire surface of theelectrode by means of a porous support plate (3), is located in thecentre as an anode (2) a titanium electrode coated with iridium oxide.The anode operates as a permanent anode. The gas-diffusion electrodecontains as the metal wire netting an expanded titanium metal coatedwith iridium mixed oxide. Between the two gas-diffusion electrodes aperiodic change of polarity takes place, such that one gas-diffusionelectrode is connected as the cathode and the other as an auxiliaryanode.

When the cell is used in electrolytic water disinfection, the limedeposited at the gas-diffusion electrode operating cathodically isdissolved again at the gas-diffusion electrode connected as an auxiliaryanode.

1.-13. (canceled)
 14. A method of electrolytically disinfecting drinkingwater comprising the steps of: contacting an anode and a cathode withwater to be disinfected, the cathode comprising a gas-diffusionelectrode including a catalyst preventing the formation of hydrogenperoxide in said water, contacting the cathode with a supply of anoxygen-containing gas, and supplying the anode and cathode with a flowof electrical current sufficient to generate disinfectant substances inthe water at the anode in sufficient quantity to make the water safe fordrinking.
 15. The method of claim 17 wherein the electrical current issupplied at a level sufficient to develop a current density of between18 and 36 mA/cm².
 16. The method of claim 17 wherein the electricalcurrent is supplied at a level sufficient to develop a free chlorinecontent of at least 33 mg/Ah.
 17. The method of claim 17 furthercomprising the steps of positioning a second cathode in the water to bedisinfected, and periodically connecting the electrical current to oneof the cathodes in such a manner as to operate the cathode as anauxiliary anode.
 18. A method of electrolytically disinfecting drinkingwater comprising the steps of: contacting an anode and at least onecathode with water to be disinfected, the at least one cathodecomprising a gas-diffusion electrode, contacting the at least onecathode with a supply of an oxygen-containing gas, supplying the anodeand at least one cathode with a flow of electrical current sufficient togenerate disinfectant substances in the water at the anode in sufficientquantity to make the water safe for drinking, and forming the at leastone cathode with sufficient catalyst to prevent the formation at thecathode of hydrogen peroxide in the drinking water.